1. Field of the Invention
The present invention relates to a gallate based complex oxide solid electrolyte material, a method of manufacturing the same and a solid oxide fuel cell.
2. Description of the Related Art
A solid oxide fuel cell (SOFC) has been constantly improved since Baur and Preis drove the SOFC at 1000xc2x0 C. in 1937 after Nernst discovered a solid electrolyte (SE) in 1899. Currently, a zirconia ceramic cell with a capacity of several kilowatts shows a driving performance of several thousand hours. Because the SOFC is usually driven at high temperature of 1000xc2x0 C. or higher, a hydrocarbon based fuel gas can be subjected to internal reforming in the cell, and that a high combustion efficiency of 60% or higher can be thereby obtained.
Typically, the SOFC is composed of a solid electrolyte and a pair of electrodes formed on both surfaces of the solid electrolyte. The electrodes are porous bodies. On the surface of one electrode, a gas containing oxygen is supplied, and on the surface of the other electrode, a gas containing hydrogen is supplied. The oxygen supplied to one electrode migrates via the solid electrolyte as oxide ions and reacts with a hydrogen component on the other electrode side to generate an electric charge and water.
Constituent materials of the SOFC must be stable in an oxidation/reduction atmosphere. In addition, since the SOFC is operated at high temperature, thermal expansion coefficients of the constituent components must be approximate from one to another, and the constituent components must be very strong and toughness. Moreover, high conductivity is required for the electrodes, and selectively high oxide-ion conductivity is required for the solid electrolyte.
Currently, as a solid electrolyte, stabilized zirconia (ZrO2) is mainly used. As a stabilizer for the zirconia, oxide of two-valence alkaline earth metal such as CaO, MgO and Sc2O3, rare earth oxide such as Y2O3 and the like are used. Ion conductivity of ZrO2 doped with CaO as alkaline earth metal is 0.01 (xcexa9cm)xe2x88x921 at 800xc2x0 C. In addition, ion conductivity of ZrO2 doped with rare earth oxide, for example, Y203, Yb2O3, Gd2O3 is about 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x922 (xcexa9cm)xe2x88x921 at 800xc2x0 C. However in this case, when the temperature is 650xc2x0 C. or lower, the ion conductivity becomes 2xc3x9710xe2x88x922 (xcexa9cm)xe2x88x921 or lower.
The stabilized zirconia added with single rare earth has been publicly known since 1970. The stabilized zirconia added with the rare earth and the alkaline earth is disclosed in Japanese Laid-Open Patent Publications Sho 57-50748 (published in 1982) and Sho 57-50749 (published in 1982).
Besides the above, as a solid electrolyte material, stabilized bismuth oxide is also used. A high temperature phase (xcex4 phase) of the bismuth oxide (Bi2O3) has a deficient fluorite structure, and exhibits a low activation energy for the migration of the oxide ions, but exhibits high oxide-ion conductivity. The high temperature phase of the bismuth can be stabilized to low temperature by dissolving the rare earth oxide thereinto, and exhibits high oxide-ion conductivity. In J. Appl. Electrochemistry, 5(3), pp. 187-195 (1975) by T. Takahashi, et al., described is that the oxide-ion conductivity of the rare metal stabilized bismuth, for example, (Bi2O3)1-X (Y2O3)X, is 0.1 (xcexa9cm)xe2x88x921 at 700xc2x0 C. and 0.01 (xcexa9cm)xe2x88x92at 500xc2x0 C., which is 10 to 100 times as high as that of the stabilized zirconia.
In Japanese Patent Publication Sho 62-45191(published in 1987), disclosed is that a mixture of the stabilized bismuth oxide and the stabilized zirconia has an oxideion conductivity of 0.1 (xcexa9cm)xe2x88x921 or higher at 700xc2x0 C. In this case, it can be expected that high ion conductivity is obtained in a temperature range lower than 1000xc2x0 C. However, since the mixture is reduced and Bi metal is deposited in a reduction atmosphere. This Bi metal deposition exhibits electronic conductivity, thus making it difficult to use the mixture as a solid electrolyte.
As another solid electrolyte, there is a ceria based solid solution. Ceria (CeO2) has a fluorite cubic structure in a temperature range from room temperature to its melting point. When rare metal or CaO is added to the oxide, a solid solution is formed in a wide temperature range. This ceria based solid solution has been reported by Kudo, Obayashi, et al (J. Electrochem., Soc., 123[3] pp. 416-419, (1976)). With regard to CeO2-Gd2O3 based solid solution, which is a topic compound in the recent research, a structure thereof is represented as Cel1-X GdXO2-X/2, where oxide vacancies are formed. Since the valence of Ce is varied in this CeO2-Gd2O3 based solid solution, the solid solution is reduced to Ce metal in a reduction atmosphere similarly to the bismuth base, and exhibits the electronic conductivity. Accordingly, it is difficult to use the solid solution as a solid electrolyte.
As still another solid electrolyte material usable at low temperature, there is a perovskite compound, on which research and development have been conducted. The perovskite compound is typically represented by a chemical formula ABO3, which includes, for example, Ba(Ce0.9Gd0.1)O3, (La0.9Sr0.1)(Ga0.8Mg0.2)O3, (Ca0.9Al0.1)TlO3, Sr(Zr0.9Sc0.1)O3 and the like. Moreover, with regard to the (La1-XSrX) (Ga1-yMgy)O3 based perovskite compounds have been reported in J. Am. Chem. soc., 116, pp. 3801-3803 (1994) by T. Ishihara, et al. and Eur. J. Solid State Inorg. Chem. t. 31, pp. 663-672 (1994) by M. Feng and J. B. Goodenough. Each of the compounds is expected to exhibit high oxide-ion conductivity in the oxidation-reduction atmosphere at low temperature.
Since an output of a single cell is just few volts, the conventional cell must be constructed in a laminated structure in order to obtain a high voltage. The laminated ceramic cell thus constructed becomes large in size, thus making a system designing difficult. Therefore it is desire to use a metal part such as ferrite based stainless steel for a vessel of a combustor body.
Accordingly, it is required to develop a solid oxide fuel cell operatable at low temperature of about 600 to 700xc2x0 C. at which a stainless steel material can be used. Also for the solid electrolyte material, selective high oxide-ion conductivity at low temperature is desired.
For example, the zirconia based solid electrolyte material conventionally used as a main solid electrolyte exhibits low oxide-ion conductivity at low temperature. Meanwhile, the bismuth or ceria based solid electrolyte material is apt to be reduced, and the electronic conductivity thereof is increased by the reduction. Therefore, both of the materials are not suitable for the solid electrolyte for the fuel cell.
Meanwhile, the gallate based perovskite compound material exhibits superior oxide-ion conductivity at low temperature as compared with the other compounds. However at low temperature, the electronic conductivity is increased as well as the oxide-ion conductivity, leading to the exhibition of the mixed electric conductivity. Therefore, there occurs a problem that a ratio of relative oxide-ion conduction, that is, a transport number, is lowered.
Accordingly, an object of the present invention is to provide a solid electrolyte material in which oxide-ion conductivity is stable and high even at low temperature, particularly to provide a gallate based complex oxide material that has the stable and high oxide-ion conductivity at low temperature.
Another object of the present invention is to provide a method of manufacturing the gallate based complex oxide material.
Still another object of the present invention is to provide a solid oxide fuel cell operatable at low temperature.
A first aspect of the present invention provides a solid electrolyte material that comprises an A site-deficient complex oxide represented by a chemical formula A1. xcex1BO3-xcex4, in which the B site contains at least Ga.
In accordance with the solid electrolyte material of the present invention, because the B site contains Ga in the solid electrolyte material, high oxide-ion conductivity appears at low temperature. Moreover, by adopting the A site-deficient structure, the oxide-ion conductivity is further improved, and the electronic conductivity is suppressed. Furthermore, the A site-deficient structure makes the crystal structure more flexible and makes the structure stabilized, thus making it possible to enhance the toughness and the durability.
A second aspect of the present invention provides a method of manufacturing the solid electrolyte material that comprises mixing oxide materials of respective constituent elements; baking temporarily the mixed materials at 1100 to 1200xc2x0 C. for 2 to 10 hours; grinding the temporarily baked materials; molding the ground materials; and sintering the molded materials.
In accordance with the method of manufacturing of the present invention, the solid electrolyte material of the present invention containing the A site-deficient complex oxide represented by the chemical formula A1-xcex1BO3-xcex4 can be prepared.
A third aspect of the present invention is a solid oxide fuel cell that comprises a solid electrolyte using the solid electrolyte material having the above-described first aspect; a cathode electrode formed on one surface of the solid electrolyte; and an anode electrode formed on the other surface thereof.
In accordance with the solid oxide fuel cell of the present invention, the solid oxide fuel cell stably operatable at low temperature can be provided.